Isn't that the entire point here? The OP wants to ensure that whatever he designs, he reaches the widest possible audience, so he wants an older system to do design testing, and to ensure that older systems can view and use what he creates.
That is precisely what I do: Why limit myself to a 30-to-55% share of the market, when all of this ultra-modern stuff is unnecessary, slows down loading time, engulfs system resources, and generally makes it impossible to use older systems to view your website.
One of my mates has an American Express card, and I couldn't load a basic banking website to make an emergency payment for her; yet, the website for my bank works on my 10.3 Systems, AndroidOS, PalmOS (on my Treo), and goodness knows what else.
The day that Yahoo Mail forces me to switch to their new UI, I will stop using their webmail. Forcing people to use newer browsers, and operating systems, limits your customer base, and is quite frankly, extremely rude.
It may shock you, but older browsers can be less susceptible to attacks, because they can't execute some of the runtime code in scripts that may contain malware. On Linux, I run NoScript (and other tools), that make it nearly impossible to use many 'new' websites. I enable the absolute, bare minumum of site elements to use these websites.
Isn't the most important factor in a website, that people can actually load it?
Many of my websites even work in Lynx, as I ensure that I tag graphics for text-only browsers.
I say, if you want to ensure compatibility with older systems, you are a fine person, who deserves a great deal of respect, for displaying a level of ethical integrity, rather than hopping on a railway to nowhere, for the sake of 'mod' features and functions, that you can do with older types of code, in exactly the same manner, that are ultimately more user-friendly.
the new YM, for example, can no-longer open each message in its own tab. We call that kind of modification: Decontenting.